1. Technical Field
The present invention relates to an optical module comprising a Faraday rotator, which is an essential component of an optical isolator that is used in a wavelength region of about 1 μm.
2. Background Art
Conventionally, industrial laser machines that are used in applications such as cutting, welding, or marking employ a CO2 laser (wavelength 10.6 μm) or a lamp pumped YAG laser (wavelength 1 μm).
In recent years, the requirements for machining performance have become more strict, and there has been a desire for a laser machine having higher precision, higher output, and longer life. Among such market requirements, attention is being focused on fiber lasers. A fiber laser is characterized by all optical paths being formed from optical fiber and by being capable of fiber-outputting laser light having high precision and high output by amplifying 1-μm-band light emitted from a laser diode (LD) light source using fiber doped with a rare-earth element such as ytterbium (Yb). Compared with a lamp pumped YAG laser with the same wavelength band, a fiber laser has high conversion efficiency for excitation light, excellent heat dissipation so that it can be air-cooled, and does not require pumping by a lamp, and it is therefore attracting attention because of merits such as low power consumption, higher output, and longer life.
However, although a fiber laser is characterized by a narrow light emission spectrum and excellent conversion efficiency, it is very sensitive to returning light due to reflected light, its properties become unstable if reflected light returns from an end face bonded to the optical fiber or from a metal surface having high reflectance and, consequently, this causes the possibility that the LD light source unit might be damaged due to high output light emission. Therefore, in order to guarantee stable operation of a fiber laser, it is essential to block light that has been reflected and is returning to the light emitting source from the optical fiber by disposing an optical isolator that has the function of transmitting light in the forward direction and blocking light in the reverse direction between the light emitting source and a workpiece for the purpose of preventing reflected light from returning to a light-emitting device, which is the light emitting source.
Here, the optical module is formed from two essential components, that is, a Faraday rotator and a magnet that applies a magnetic field in the direction of light transmission of the Faraday rotator (optical axis direction). When light is incident on the Faraday rotator in this configuration, a phenomenon occurs in which the plane of polarization rotates in the Faraday rotator. This is a phenomenon called the Faraday effect; the angle through which the plane of polarization rotates is called the Faraday rotation angle, and its magnitude θ is represented by the equation below.θ=V×H×L wherein V is the Verdet constant, which is a constant determined by the material of the Faraday rotator and measurement wavelength, H is the magnetic flux density, and L is the length of the Faraday rotator.
As can be understood from this equation, in order to obtain a desired Faraday rotation angle in a rotor having a certain fixed magnitude for the Verdet constant, the larger the magnetic field applied to the Faraday rotator the shorter the length of the rotator can be, and the longer the length of the rotator the smaller the magnetic flux density can be.
As a material for the Faraday rotator, an oxide having a specific composition has been disclosed in JP-A-2010-285299 (JP-A denotes a Japanese unexamined patent application publication).
An optical isolator is constituted by disposing a pair of polarizers on the side on which light is incident and the side on which light emerges from the above-mentioned optical module, and in order to have a function as an optical isolator, in general a Faraday rotation angle of on the order of 45° is necessary. Specifically, the plane of polarization of light that enters the optical isolator is rotated through 45° by the Faraday rotator, and the light is transmitted through the incident/emergent polarizers whose angles have been adjusted individually. On the other hand, the plane of polarization of returning light is rotated through 45° in the opposite direction by utilizing the nonreciprocity of the Faraday rotator to thus give an orthogonal plane of polarization that is at 90° to the incident polarizer, and the light cannot be transmitted. The optical isolator utilizes this phenomenon, allows light to be transmitted only in a single direction, and blocks light that has returned after being reflected.